The present invention relates to an apparatus for measuring magnetic fields which change with an only extremely low frequency by means of a SQUID magnetometer which comprises a superconducting flux transformer for inductively coupling the measuring signal into a d-c SQUID sensor.
With SQUIDs (superconducting quantum interference devices), magnetometers with high sensitivity to magnetic fields, for instance, in the order of 10.sup.-14 Tesla (Hertz).sup.-1/2 can be constructed. Such a magnetometer contains in general, besides a SQUID sensor, also a flux transformer, with which the magnetic fields to be detected can be coupled into the SQUID sensor.
In such magnetometers, the outermost limit for their sensitivity is determined by the so-called 1/f noise, i.e., inversely proportional to the frequency (see, for instance, "Journal of Low Temperature Physics," Vol. 51, nos. 1/2, 1983, pages 207 to 224).
In the publication "SQUID-Superconducting Quantum Interference Devices and Their Applications" (Proc. Int. Conf. on Superconducting Quantum Devices, Berlin, 1976), 1977, pages 439 to 484, a device for underwater communication can be seen, with which also magnetic fields with extremely low frequencies of 30 to 3000 Hz can be detected. This device comprises, among other things, an RF SQUID as a sensor which is arranged together with a superconducting flux transformer in a cryostat (see page 449, FIG. 3). This flux transformer comprises a field coil, also called a detection loop, which serves as an antenna and is connected to a coupling coil for coupling the measuring signal into the SQUID. These superconducting parts of the device can be realized, in particular, as a thin-film structure. The superconducting part of the device formed by the flux transformer and the SQUID sensor is followed by low-noise electronic circuitry.
For further reducing the noise, it is known in addition, to use d-c SQUIDs for such measuring devices (see, for instance "Applied Physics Letters," Vol. 40, no. 8, Apr. 15, 1982, pages 736 to 738).
If, however, quasi static magnetic fields with frequencies of, for instance, 0.01 Hz with measuring times in the range of seconds are to be measured, the known devices could not be used up to now because of the mentioned l/f noise of the SQUID sensor which sets in below about 1 Hz. This applies particularly to fields from remote sources which cannot be influenced, for instance, modulated, and which have very little inhomogeneity. Special magnetometers can also not be used, in which a modulated signal is generated by the vibration of a detection coil. Similarly to this procedure, the modulation can also be brought about by the motion of a magnetic probe relative to a detection coil (see, for instance, the book by B. I. Bleaney and B. Bleaney: "Electricity and Magnetism," Third Edition, 1976, page 187). For the mentioned extremely low frequency range, the characteristic length of the field variation to be generated in this manner would then have to be much larger than the dimensions of the SQUID sensor and its coolant container surrounding it.